Different supply voltages (V.sub.DD) have been used in computer systems to accommodate different types of transistor technologies of the logic circuit comprising the computer. The popularity of portable battery powered computers has created a need for lower system operating voltages and corresponding reductions in power consumption. CMOS transistor technology may operate at various voltage levels, for example, 5 Volts or 3.3 Volts. However, at lower voltage levels, the speed and performance of CMOS logic circuits may be degraded. Typically, a CMOS circuit may be only 70% as fast when supply voltage V.sub.DD is set at 3.3 Volts versus 5 Volts.
For a video controller integrated circuit (IC), such as a VGA controller IC, it may be desirable to operate the IC at a lower supply voltage V.sub.DD in order to reduce power consumption, for example, when applied to a laptop or notebook computer. In addition, with the advent of low power consumption desktop computers and the like, it may be desirable to operate circuitry within a personal computer (PC) at a lower operating voltage if possible.
For a video controller IC, however, performance limitations may prevent the IC from being operated at a low voltage. For example, a VGA controller IC may be required to support one or more pixel resolutions, such as 640.times.480, 800.times.600, or 1024.times.768, a number of pixel depths, such as 8, 16, or 24 bits per pixel (BPP), and a number of refresh rates, such as 60 Hz, 72 Hz or 75 Hz. A video controller operating at a lower supply voltage V.sub.DD (e.g., 3.3 volts) may be able to operate at a memory clock speed sufficient to provide adequate data bandwidth to support lower resolution displays at lower refresh rates (e.g., 640.times.480 @ 8 BPP resolution at 60 Hz) but may not have enough data bandwidth due to reduced MCLK frequency to support higher resolutions at higher refresh rates (e.g., 800.times.600 @ 16 BPP @ 75 Hz). The maximum memory clock frequency at 3.3 Volts may not allow enough memory bandwidth for the controller to support higher resolution modes.
One solution to such a problem is to configure an IC such as a video controller IC to operate at a predetermined voltage such that the highest contemplated resolution (i.e., required data bandwidth) is supported by that predetermined voltage. While such a technique may insure that the IC will operate in desired video modes, power savings are not maximized when the IC is operating in lower resolution modes. For example, if an IC is configured to operate at 5 Volts to support 1024.times.768 pixel resolution, potential power savings may be lost if the IC is operated at lower resolutions (e.g., 640.times.480) for substantial periods of time.